Adam Seddon said:

Is that correct or not?

So following on from that, my question is: why cant we just isolate all transformers from earth and remove the risk of a "line to earth" shock? After all, this is exactly why we use SELV transformers in bathrooms no???? 

Obviously you could still get a line to neutral shock, but that's got nothing to do with the question I'm asking here. 

Yes, you are correct, that is how electrical separation and IT system works.

The problem with an unearthed system, is that on the first fault of the system to something conductive connected to Earth goes undetected ... making it a TN system effectively, and it's thus the second fault that kills.

Hence the additional requirements in BS 7671 for IT systems ... and limitation of 1 piece of equipment supplied by electrical separation (unless the installation is under constant supervision of skilled and/or instructed persons).

I still wonder about the actual science behind perceived electric shocks,at college(in Stevenage) we was told its the resistance between you and the mass of earth , but in my thick rubber sole boots on a carpet on a wooden floor this can’t be true.

i have measured a few pico farads between me and earth (though this was measured at 1000 hertz) so I assume this is the current path?

The exact current path is a mixture of the parallel paths of conduction (resistive) and displacement current (a catch all term for the capacitances , where the body is one plate, and the conductive bits of the surroundings form the various  other 'plates' and the dielectric filler in that capacitor is the air, and/or the insulating bits of the local environment)

You are quite right, on a dry wood floor, the main effect will be displacement  - and unless you are very slim I'd expect you to be a few hundred pF to the 'plate at the end of the universe' and at 50Hz the current will be just about enough to light a neon screwdriver or operate a volt stick, but not really enough to be more than a very weak sensation, maybe not perceptible.

So with well insulated footwear and taking care to be nowhere near an earthed radiator or anything like that, one can safely touch a live terminal and feel nothing.  BUT unless you are in the centre of the room and standing on an upturned  plastic bucket or something you do not know for sure if there is an earthed object nearby or under the floor that totally changes all that and makes the capacitance so much greater as to be a serious hazard.

But in leather soled shoes or indeed any footwear that is a bit damp, especially on a concrete floor without DPC, or on the ground outside, the effect is mostly conduction, and as many will attest jolly painful ....

The worst shocks are with two clear metallic contact,  usually one live and one earth or neutral. (occasionally the other phase)  Then there is a defined current path, burns and all the usual physiological effects, and in  extreme cases, death.

Mike

The exact current path is a mixture of the parallel paths of conduction (resistive) and displacement current (a catch all term for the capacitances , where the body is one plate, and the conductive bits of the surroundings form the various  other 'plates' and the dielectric filler in that capacitor is the air, and/or the insulating bits of the local environment)

You are quite right, on a dry wood floor, the main effect will be displacement  - and unless you are very slim I'd expect you to be a few hundred pF to the 'plate at the end of the universe' and at 50Hz the current will be just about enough to light a neon screwdriver or operate a volt stick, but not really enough to be more than a very weak sensation, maybe not perceptible.

So with well insulated footwear and taking care to be nowhere near an earthed radiator or anything like that, one can safely touch a live terminal and feel nothing.  BUT unless you are in the centre of the room and standing on an upturned  plastic bucket or something you do not know for sure if there is an earthed object nearby or under the floor that totally changes all that and makes the capacitance so much greater as to be a serious hazard.

But in leather soled shoes or indeed any footwear that is a bit damp, especially on a concrete floor without DPC, or on the ground outside, the effect is mostly conduction, and as many will attest jolly painful ....

The worst shocks are with two clear metallic contact,  usually one live and one earth or neutral. (occasionally the other phase)  Then there is a defined current path, burns and all the usual physiological effects, and in  extreme cases, death.

Mike